1. Field of the Invention
The present invention generally relates to a method for inspecting a disconnection in an electric circuit of a wiring board, more particularly, to a method for inspecting a disconnection in an electric connection between electric circuits formed on both surfaces of a printed wiring board.
2. Description of the Related Art
Recently, in guaranteeing a quality of a printed wiring board, a probe-moving inspecting method is widely applied, in which method a probe is brought into contact with the printed wiring board so as to inspect a disconnection in an electric wiring one by one. An inspection like this is required to be highly reliable.
Other conventional disconnection-inspecting methods include a method of contacting a probe on one surface of a printed wiring board, as follows. That is, in this method, first, a printed wiring board is placed on an insulating sheet laid on a reference conductor, and then a probe is brought into contact with a measurement pad on a circuit formed on the printed wiring board. Next, electric signals are supplied to the reference conductor and the circuit so as to measure an electric voltage and an electric current generated between the reference conductor and the circuit. Thereby such values as a capacitance are obtained so as to judge the presence of a disconnection in the electric circuit.
Conventional disconnection-inspecting methods also include, as a method for inspecting a disconnection in electric circuits formed on both (upper and under) surfaces of a printed board and connected to each other, a method of obtaining a capacitance value in the above-mentioned manner for each of a plurality of printed boards having a same circuit, and then estimating a disconnection from a distribution of the capacitance values by a majority method.
Conventional disconnection-inspecting methods also include a method of overturning such a printed board as above on a reference conductor so as to obtain capacitance values between each of circuits formed on both surfaces of the printed board and the reference conductor and then judging that a disconnection does not exist if the two obtained capacitance values are substantially equal.
However, there are problems that the above-mentioned conventional methods cannot find all-product inferiority in which a plurality of printed boards incur a same defect due to such a cause as a fault of a mask used in forming a circuit, and that the above-mentioned conventional methods cannot judge the presence of a disconnection in a case where there is only one printed board as a target of a disconnection inspection.
Additionally, in the above-mentioned methods including obtaining capacitance values as to both surfaces of a printed board, when a thickness of an insulating sheet is made so large that both capacitance values become equivalent, the capacitance measurement is required to be precise. Further, there is a problem in an inspection reliability that, in a case where areas of patterns of circuits on both surfaces are substantially equal, no difference is recognized in capacitance values on the upper and under surfaces, and consequently a disconnection is overlooked.
It is a general object of the present invention to provide an improved and useful disconnection-inspecting method in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a simple and reliable disconnection-inspecting method for inspecting a disconnection regarding an electric circuit.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a disconnection-inspecting method for inspecting an electrical disconnection between circuits formed on both surfaces of a board, the method comprising the steps of:
placing the board on an insulating sheet laid on a reference conductor;
measuring a first capacitance between the reference conductor and one of the circuits formed on a surface of both surfaces opposite to the other surface facing the insulating sheet;
measuring a second capacitance between the reference conductor and the one of the circuits by changing a first physical quantity of the insulating sheet;
calculating a second physical quantity of each of the circuits based on the first capacitance and the second capacitance measured in the steps of measuring; and
judging the presence of the electrical disconnection based on the second physical quantity calculated in the step of calculating.
According to the present invention, since an electrical disconnection is judged to be present or not based on the second physical quantity calculated from the measured capacitance values, the presence of an electrical disconnection in electric circuits can easily and surely be inspected. Additionally, since the present disconnection-inspecting method does not use a statistical method, the presence of a disconnection can be judged even in a case where there is only one board as a target of the inspection.
More specifically, in the disconnection-inspecting method according to the present invention, the first physical quantity may be a thickness or a dielectric constant of the insulating sheet. Additionally, the second physical quantity may be an area of each of the circuits so that the step of judging includes judging the presence of the electrical disconnection according to whether or not the area is calculated as 0.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention a disconnection-inspecting method for inspecting an electrical disconnection between circuits formed on both surfaces of a board, the method comprising:
the first step of placing the board on an insulating sheet laid on a reference conductor;
the second step of measuring a first capacitance between the reference conductor and one of the circuits formed on a surface of both surfaces opposite to the other surface facing the insulating sheet;
the third step of measuring a second capacitance between the reference conductor and the one of the circuits by changing a first physical quantity of the insulating sheet;
the fourth step of calculating a second physical quantity of each of the circuits based on the first capacitance and the second capacitance measured in the steps of measuring;
the fifth step of overturning the board on the insulating sheet and repeating the second step to the fourth step; and
the sixth step of judging the presence of the electrical disconnection according to whether or not the second physical quantity calculated in the fourth step and the second physical quantity calculated in the fifth step are equal.
According to the present invention, even in a case where circuits formed on both surfaces of a board are electrically connected by a plurality of leading wires and some of the leading wires have disconnected parts, the presence of the disconnection in the leading wires can surely and easily judged assuming that areas of patterns of circuits are different on both surfaces of the board.
More specifically, in the disconnection-inspecting method according to the present invention, the first physical quantity may be a thickness or a dielectric constant of the insulating sheet. Additionally, the second physical quantity may be an area of each of the circuits.